Selective Catalytic Reduction (SCR) of NOx using urea as a reductant is well established for NOx emissions reduction on stationary sources and mobile applications. In the SCR process, NOx reacts with a reductant, such as pure anhydrous ammonia, aqueous ammonia, and/or urea, which is injected into the exhaust gas stream before a special SCR Catalyst. The SCR approach significantly reduces diesel NOx.
The SCR process requires precise control of the reductant injection rate. Insufficient injection may result in unacceptably low NOx conversion. An injection rate that is too high may release reductant to the atmosphere. The current dosing control system uses open loop dosing maps, based on engine speed and load, with temperature modifiers to lookup the required dosing quantity. However, the open loop controller logic may not result in optimum NOx emissions elimination, perhaps due to transient operation with low levels of emissions. Due in part to minimum practical settings on reductant dosing mechanisms, it is difficult to precisely supply reductant at these low levels without slippage of reductant into the atmosphere.